/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/calls/calls.h"
#include "libc/errno.h"
#include "libc/macros.h"
#include "libc/mem/mem.h"
#include "libc/stdio/internal.h"
#include "libc/stdio/stdio.h"
#include "libc/str/str.h"
#include "libc/sysv/consts/o.h"

/**
 * Repositions open file stream.
 *
 * This function flushes the buffer (unless it's currently in the EOF
 * state) and then calls lseek() on the underlying file. If the stream
 * is in the EOF state, this function can be used to restore it without
 * needing to reopen the file.
 *
 * @param f is a non-null stream handle
 * @param offset is the byte delta
 * @param whence can be SEET_SET, SEEK_CUR, or SEEK_END
 * @returns 0 on success or -1 w/ errno
 */
int fseek_unlocked(FILE *f, int64_t offset, int whence) {
  int res;
  int64_t pos;
  if (f->fd != -1) {
    // handle file streams
    if (fflush_unlocked(f) == EOF)
      return -1;
    if (whence == SEEK_CUR && f->beg < f->end)
      offset -= f->end - f->beg;
    if (lseek(f->fd, offset, whence) != -1) {
      f->beg = 0;
      f->end = 0;
      res = 0;
    } else {
      f->state = errno == ESPIPE ? EBADF : errno;
      res = -1;
    }
  } else {
    // resolve absolute position
    switch (whence) {
      case SEEK_SET:
        pos = offset;
        break;
      case SEEK_CUR:
        pos = f->beg + offset;
        break;
      case SEEK_END:
        pos = f->end + offset;
        break;
      default:
        pos = -1;
        break;
    }
    if (pos < 0) {
      // can't seek before file
      f->state = errno = EINVAL;
      res = -1;
    } else if (f->memstream_bufp) {
      // handle open_memstream() streams
      if (pos + 1 > f->size) {
        char *buf2;
        size_t size2;
        size2 = f->size;
        do {
          size2 += size2 >> 1;
        } while (pos + 1 > size2);
        if (!(buf2 = realloc(f->buf, size2)))
          return EOF;
        memset(f->buf + f->size, 0, size2 - f->size);
        *f->memstream_bufp = f->buf = buf2;
        f->size = size2;
      }
      f->end = MAX(pos, f->end);
      f->beg = pos;
      res = 0;
    } else {
      // handle fmemopen() streams
      f->end = MAX(f->beg, f->end);
      if (0 <= pos && pos <= f->size) {
        f->beg = pos;
        res = 0;
      } else {
        f->state = errno = EINVAL;
        res = -1;
      }
    }
  }

  // A successful call to fseek() shall clear the end-of-file
  // indicator for the stream and undo any effects of ungetc() and
  // ungetwc() on the same stream. After an fseek() call, the next
  // operation on an update stream may be either input or output.
  if (!res)
    f->state = 0;

  return res;
}
